1. Field of the Invention
The present invention relates to a rectangular battery having a spiral type electrode unit housed in a rectangular exterior casing.
2. Description of the Related Art
The electrode unit of a rectangular battery laminates a separator between a positive electrode and a negative electrode. There are different types of layered structures of positive and negative electrodes. There is a sandwich configuration where positive and negative electrodes are cut in square shapes and stacked with the separator between them. There is also a spiral type electrode where long narrow strips of positive and negative electrode are rolled into a spiral shape with the separator between them and then pressed from both sides to flatten the sides (refer to Japanese Patent Laid-Open Publication 2001-266946 and Japanese Patent Laid-Open Publication HEI 9-171809 (1997)).
The spiral electrode rectangular battery cited in these and other disclosures has the characteristic that it can be efficiently manufactured in quantity while thin positive and negative electrodes can be rolled many turns to reduce the battery's internal electrical resistance. Consequently, this type of rectangular battery is preferably used in lithium ion rechargeable batteries, which have thin electrode plates. Since an electrode unit of this configuration is made by pressing both sides of the unit after rolling it into a spiral shape, it is preferable to position the end of the outer-most electrode plate winding at a rounded edge region rather than on a flattened surface. An electrode unit with the electrode end located at a rounded edge region allows both sides to be pressed into ideally flattened surfaces, Otherwise if the outer-most electrode plate end is located on a flattened surface, the surface will be pressed with a step formed along the boundary at the end of the outer-most electrode plate.
Further, as cited in Japanese Patent Laid-Open Publication H09-171809, a cut-out is provided on the flattened surface of the outer-most electrode plate, and this cut-out electrode region is folded-back for use as a collector terminal. Accordingly, in this type of electrode unit with the end of the outer-most electrode located at a rounded edge region, the collector terminal can be made sufficiently wide. A wide collector terminal has the characteristic that its internal electrical resistance is low and it can be reliably connected to the exterior casing.
However, a rectangular battery that positions the end of the outer-most electrode plate at a rounded edge region has the drawback that the end of the outer-most electrode plate becomes prone to breaking through the separator and causing an internal short circuit. In particular, if the end of the outer-most electrode plate is rolled off-center with a very slight shift up or down, a sharp edged corner between the sides and end of the outer-most electrode plate can tear through the separator, or particles and dust can ingress between the positive and negative electrodes in a corner region, and pierce through the separator to make internal short circuits likely. In addition to this drawback, if the rectangular battery is dropped and a corner of the exterior casing is impacted, the exterior casing can induce mechanical shock at a corner of the electrode unit. This also has the detrimental result that a corner region of the outer-most electrode plate becomes prone to breaking through the separator to cause an internal short circuit.
The present invention was developed with the object of further resolving the drawbacks described above. Thus it is a primary object of the present invention to provide a spiral electrode rectangular battery that, in addition to allowing ideal flat surfaces on both sides of the electrode unit, can effectively prevent internal short circuits at corner regions on the end of the outer-most electrode plate. The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.